1. Field of the Invention
The present invention relates to electrolytic cells and more particularly to systems such as fuel cells, batteries, industrial electrolysis equipment and the like.
2. Description of the Prior Art
U.S. Pat. No. 4,192,907 of Jalan et al for "Electrochemical Cell Electrodes Incorporating Noble Metal - Base Metal Alloy Catalysts" suggests a fuel cell with a substrate of an electrically conductive material coated with a platinum base alloy. Preferably the alloy is coated on a substrate comprising conductive particles of carbon. This electrode deteriorates because of the segregation of the component elements in the alloy on the surface of the alloy with small clusters of each of the elements from the alloy forming on the surface of the alloy. Whereas Pt alone is a catalyst, the refractory metal added by Jalan et al to the Pt to form the alloy is ineffective (when isolated into islands or when in the bulk form) as a catalyst. That is, the refractory metal affords little or no catalytic activity. In addition, the Jalan et al alloy would be less expensive than Pt alone which is an important reason for trying to use it, but its life would not be extended beyond the life of Pt because of segregation and impurities. The impurities referred to are unwanted species deposited on the catalyst from the bath.
It is known that a layer deposited at an underpotential referred to hereinafter as an underpotential deposited layer alters the electronic properties of the deposited metal making it more noble than the bulk metal, based on material published in a review article written by D. M. Kolb, published in Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Gerischer and Tobias, Eds., Interscience (1977), 125-271, especially pages 234-239. There was no suggestion that the subject matter of the article could be used as a catalyst in the way described in this application.
Lead has been deposited on gold by underpotential deposition as described by McIntyre, et al, "Electrochemical Catalysis by Foreign Metal Adatoms", Abstract 339, Electrochemical Society pages 864-5 (1979). However, this was not done in connection with a catalyst for an electrolytic cell.
Copper was employed on gold in an alkaline solution, as described in Amadelli, Bindra, Khutornoi and Yeager, "Influence of Metal Ions on the 02 Reduction of Noble Metals in Alkaline Solutions", Extended Abstracts of the ECS Spring Meeting, (1979) Boston, Mass.
Recent advances in the search for new oxygen reduction electrocatalysts has led to the use of UPD monolayers of metals deposited at an underpotential to produce electrocatalytic surfaces of substantially enhanced activity and stability. See Khutornoi et al supra, and McIntyre et al supra. Formation of a UPD layer alters both the geometric structure of the surface as well as the electronic properties of the deposited metal rendering it more noble than the bulk metal. See Kolb et al supra. The electronic properties of the deposited metal are often quite specific to the UPD layer in combination with a specific substrate. Thus these modified electrodes also provide rather convenient and useful model systems for studying the correlations between the catalytic behavior and the surface electronic and geometric structure.
The electrocatalytic activity of several UPD systems for the reduction of oxygen gas in alkaline and acid media has been examined by McIntyre et al supra and the work of Amadelli, Molla, Bindra, and Yeager, J. Electrochem. Soc., supra, in the alkaline case; and Bindra, Clouser et al in the acid case. There is an apparent anomaly in the behavior of the UPD layers in the two media. In an alkaline solution, the diffusion limiting current for oxygen reduction on gold in the presence of UPD layers of metals such as Tl, Pb (McIntyre et al supra) and copper (Amadelli et al supra) is almost doubled, indicating a favoring of the four electron reduction. Where there is an electrode surface of gold covered with a UPD layer of a catalytic metal, the result is that the equation of the chemical reaction involved is as follows: EQU O.sub.2 +2H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.-
In the case of a surface of a metal such as gold in bulk form, in the absence of the UPD species the result is that the chemical reaction is different as follows: EQU O.sub.2 +H.sub.2 O+2e.sup.- .fwdarw.HO.sub.2.sup.- +OH.sup.-
The net effect of this is that we have observed that twice as much power can be realized per oxygen molecule.
Even on Pt substrates, oxygen reduction current in alkaline solution has been shown to increase in the presence of trace quantities of Tl positive ions in the electrolyte as described by Amadelli, Bindra, and Yeager, supra.
In acid media, on the other hand, the UPD of copper seems to lower the catalytic activity of Pt for oxygen reduction as described in Bindra, Clouser et al supra. In an effort to understand the anomalous behavior the catalytic effects of UPD layers on oxygen reduction have been examined in weakly acidic, neutral, and alkaline electrolyte.
Lead has been deposited on mercury or gold in a neutral medium (pH of about 7) in the form of a UPD layer of lead on mercury and lead on gold in a paper by Bindra, "The Effect of pH on the Electrocatalytic Properties of Adsorbed Metal Ions", Journal of the Electrochemical Society (submitted 1982).
Lead or thallium on gold has been published by Amadelli et al "Influence of Metal Ions on the 02 Reduction of Noble Metals in Alkaline Solutions", Journal of the Electrochemical Society, Vol. 128, No. 12, pages 2706-2709 (December 1981).
This invention is believed to differ from the prior art in that it has several novel features.
Firstly, the system of this invention utilizing an element selected from the groups consisting of:
(a) Pt, Ir, Pd, Ag, or Rh as a catalyst in alkaline media and to control impurities in any media, or
(b) Pb, Tl or Bi as a catalyst in alkaline media or as a means to control impurities in acid media on thin crystallites of Au. They are distinguished from the above systems in that the substrate is particulate and some of the adsorbates are noble metals. In the above systems the adsorbate is a base metal such as lead.
Another aspect of this invention resides in the novel feature of using a catalyst such as Pt rather than Pb where the Pb would poison one of the electrodes in the electrochemical system. That statement is qualified by the fact that a diaphragm between electrodes in the system could be used to avoid poisoning, but that the cost of such a solution would be reduced voltage. It should be understood that as a catalyst alone, Pb is effective in those cases in which it would not tend to poison the electrodes.
Secondly, small crystallites of gold and a monolayer of a UPD catalyst metal in solution provides (a) a high surface area, (b) the UPD layer can stay only on the surface of the substrate metal (gold) since the substrate metal is too thin or not bulky enough for the catalyst layer to diffuse into the bulk (i.e., thick film metal).
Thirdly, UPD metal catalysts in this environment control the buildup of impurities on the surface of the catalyst which is not described in any of the above references. Here we refer to metals such as Pt, Pd, Ag, Rh, Ir, as well as Pb, Tl, and Bi in the case of acid media (electrolytes).